1. Field of the Invention
This invention relates to a pulley which is used for transmitting driving force by belt.
2. Discussion of the Background
As for a pulley which is made of synthetic resin, a metallic insert is fitted into the center portion of a disk as a main body of the pulley in order to transmit rotational driving force between the disk and a shaft. Then, it is necessary that the metallic insert is strongly secured in order to bear external force of axial direction. In order to achieve the above two purposes, i.e. to transmit the rotational driving force and to bear the external force, a first conventional metallic insert 120 shown by FIG. 1(a) has a concavity 121 which is formed on the center in the axial direction of the periphery surface of the metallic insert 120 and serrations 122 which are formed on each side of the concave 121. A second conventional metallic insert 130 shown by FIG. 1(b) has a concavity 131 and serrations 132 which are formed on the bottom surface of the concavity 131. However, it is difficult for the serrations 132 to be machined at the second conventional metallic insert 130. In contrast, a third conventional metallic insert 140 shown by FIG. 1(c) and a fourth conventional metallic insert 150 shown by FIG. 1(d) have a convexity 141, 151 which is formed on the center in the axial direction of periphery surface of the metallic insert 140, 150. Serrations 142, 152 are formed on the periphery surface of the convexity 141 at the third conventional metallic insert 140 and serrations 152 are formed on each side of the convexity 151 at the fourth conventional metallic insert 150. However, burrs produced by machining the serrations 142 damage the disk when the metallic insert 140 is fitted into the disk, and it is possible that strength of the pulley deteriorates at the third conventional metallic insert 140. It is also difficult for the serrations 152 to be machined at the fourth conventional metallic insert 150. A fifth conventional metallic insert 160 shown by FIG. 1(e) and a sixth conventional metallic insert 170 shown by FIG. 1(f) have a step 161, 171 on the periphery surface. Serrations 162, 172 are formed on a higher portion of the step 161, i.e. larger diameter portion, at the fifth conventional metallic insert 160 and formed on a lower portion of the step 171, i.e. smaller diameter portion, at the sixth conventional metallic insert 170. The fifth and sixth conventional metallic insert 160, 170 can bear the external force from one direction, however cannot bear the external force from another direction. Therefore, the first conventional metallic insert 120 is the most suitable in viewpoint of above purposes. However, as shown by FIG. 2, a left and a right side surfaces 121a of the concavity 121 are parallel each other, and synthetic resin as a material of the disk gets into and fills with the concavity 121 when the metallic insert 120 is fitted into a boss 111 of the disk 110. The synthetic resin filled into the concavity 121 is constructing a protrusion 115. Since the synthetic resin as the material for the disk 110 has linear expansion coefficient which is larger than the same of metal such as iron as the material of the metallic insert 120, the boss 111 thermally expands more than the metallic insert 120 when temperature rises. As the result, the protrusion 115 expands to directions shown by arrows Q in FIG. 2, and the side surfaces 121a of the concavity 121 work so as to restrain the expansion of the protrusion 115. That is, side surfaces of the protrusion 115 are pressed by the side surfaces 121a of the concavity 121 in directions shown by arrows P in FIG. 2. Therefore, extremely large tensile stress acts at a root of the protrusion 115 which edges E of the concavity 121 contact with. Since the tensile stress is lost when temperature drops, the protrusion 115 receive repeated load. The repeated load causes deterioration of strength of the protrusion 115 such as generation of crack in the root of the protrusion 115 during long term use.